Charging to 80% or 100%

[Dr Bob]

I sometimes hear people say about running the battery down to single digits to 'balance the cells' or something.
But I have heard others say its not relevant on a Taycan.

Fish Fingers,





Good question.....but it is not a simple answer …..groan!





Li battery banks can be balanced either at the top of the SoC range (top balanced) or at the bottom of the SoC range (bottom balanced). It is possible an intelligent BMS may attempt to balance at both ends but I'm not sure it would work. Note, as I said before, you can really only identify an in-balance at the voltage knees both top and bottom when it is clear that the cell to be balanced has a voltage too high (at the top) or too low (at the bottom). You cannot see if a cell needs balancing when it is in the plateau range 30-80% SoC.


To understand balancing we need to consider why a cell could be out of balance? There are a number of reasons but here are two. Firstly it could be a cell issue, ie a difference in internal resistance. The simple way to describe this is that cell is more difficult to charge so every time you get to 90%, that cell is a laggard. Its voltage is always lower than the others. Another reason is that a cell has a lower capacity than the rest – now this is important! Let's assume that most cells have a capacity of 10Ahr but the bad one has only 9Ahr. If you balance at the top and bring that cell up to the same average power, when you discharge and get down to low SoCs, that cell will be almost empty well before the rest – so well out of balance at the bottom. For my boat batteries with no auto balancing, I can either bottom balance and run the bank very low, or I can top balance and run the bank high. I think it is too difficult to top balance – as all these EVs do – and expect good balancing at the bottom unless the cells are very well matched. I am not an electrical engineer so don't know how the balancing is done – but on most boat systems, it only happens at the top end while the voltage is in the top knee. The wires to do the balancing (to every cell) have to be thin so dont carry much current so it takes time to do. My guess is that there is a far bit of spare capacity at the bottom of the voltage curve so that although the EVs are top balanced, there is room at the bottom when SoC gets less than 10% (reported SoC), for laggard cells to not get to 0%. This fits with the data that says that a Taycan 93kWh bank has only 80(something) kWh capacity - or there is slack at the top end so it is more difficult to overcharge


On my boat which has 12 * 150 Ahr cells in a 4S * 3P config for 12V, it is effectively four 3.3V cells in series. I always see Cell 2 as the 2nd highest in the plateau range but the laggard of the four as I get to the middle of the top knee. I expect Cell 2 (3 cells in parallel) has a lower capacity and will be easily the first to get to 0% SoC as I discharge. I therefore never go below 30% and keep the system between 30% and 80%.


Unless Porsche have some very clever electrickity guys then I cant seem them top and bottom balancing at the same time.

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[Dr Bob]

He also recommended that every month or two I should run the battery down to 10% to allow the software to 'balance' the cells at this level of charge. It will also flag potential problems you might have In the future with particular cells. In my case, all cells were reading as 'good' until they took the charge down to 10%, then they got the error message that allowed them to replace the battery module(s).

I wonder if he has used the word 'balance' incorrectly? If you have cells that have lost capacity then the top balancing that definitely happens on a Taycan will not spot them, and you wont see them in the plateau voltage. You will see them with very low voltage at low SoCs and that is likely what the engineer wants to look at to identify the bad cell. If you balance at the bottom, you have to re-balance next time you get to the top-which seems like hard work.

My guess is that the software is looking at low SoC to 'identify' bad cells to help balancing at the top end.

 

[Fish Fingers]

Fish Fingers,





Good question.....but it is not a simple answer …..groan!





Li battery banks can be balanced either at the top of the SoC range (top balanced) or at the bottom of the SoC range (bottom balanced). It is possible an intelligent BMS may attempt to balance at both ends but I'm not sure it would work. Note, as I said before, you can really only identify an in-balance at the voltage knees both top and bottom when it is clear that the cell to be balanced has a voltage too high (at the top) or too low (at the bottom). You cannot see if a cell needs balancing when it is in the plateau range 30-80% SoC.


To understand balancing we need to consider why a cell could be out of balance? There are a number of reasons but here are two. Firstly it could be a cell issue, ie a difference in internal resistance. The simple way to describe this is that cell is more difficult to charge so every time you get to 90%, that cell is a laggard. Its voltage is always lower than the others. Another reason is that a cell has a lower capacity than the rest – now this is important! Let's assume that most cells have a capacity of 10Ahr but the bad one has only 9Ahr. If you balance at the top and bring that cell up to the same average power, when you discharge and get down to low SoCs, that cell will be almost empty well before the rest – so well out of balance at the bottom. For my boat batteries with no auto balancing, I can either bottom balance and run the bank very low, or I can top balance and run the bank high. I think it is too difficult to top balance – as all these EVs do – and expect good balancing at the bottom unless the cells are very well matched. I am not an electrical engineer so don't know how the balancing is done – but on most boat systems, it only happens at the top end while the voltage is in the top knee. The wires to do the balancing (to every cell) have to be thin so dont carry much current so it takes time to do. My guess is that there is a far bit of spare capacity at the bottom of the voltage curve so that although the EVs are top balanced, there is room at the bottom when SoC gets less than 10% (reported SoC), for laggard cells to not get to 0%. This fits with the data that says that a Taycan 93kWh bank has only 80(something) kWh capacity - or there is slack at the top end so it is more difficult to overcharge


On my boat which has 12 * 150 Ahr cells in a 4S * 3P config for 12V, it is effectively four 3.3V cells in series. I always see Cell 2 as the 2nd highest in the plateau range but the laggard of the four as I get to the middle of the top knee. I expect Cell 2 (3 cells in parallel) has a lower capacity and will be easily the first to get to 0% SoC as I discharge. I therefore never go below 30% and keep the system between 30% and 80%.


Unless Porsche have some very clever electrickity guys then I cant seem them top and bottom balancing at the same time.

Thankyou once again for taking the time to write this up.
It's greatly appreciated.

 

[johnarpy]

I’m not going to fill any of my 911s only to 80% so I’m not doing it with my Taycan. The batteries limit us enough. If eighty percent is really the maximum you can generally use, then that should be the figure they advertise. Obviously you should never buy any of my used electric cars. I’ve owned 12 so far. All of them had no battery degradation.

 

[kort]

I’m not going to fill any of my 911s only to 80% so I’m not doing it with my Taycan. The batteries limit us enough. If eighty percent is really the maximum you can generally use, then that should be the figure they advertise. Obviously you should never buy any of my used electric cars. I’ve owned 12 so far. All of them had no battery degradation.

wow, a lot of bad info to unpack. what you do/did in your gasser is of no relevance to best practices for an EV. there is nothing wrong with loading the battery to 100% SOC as long as you do not let it sit for hours/days at that SOC.
charging to 100% as needed will cause no issues, letting the car sit at extremely high or low SOC is where the issues begin.

 

[Dr Bob]

there is nothing wrong with loading the battery to 100% SOC as long as you do not let it sit for hours/days at that SOC.
charging to 100% as needed will cause no issues, letting the car sit at extremely high or low SOC is where the issues begin.

I think hours/days is not good advice. Days, yes I agree that is bad. Charging overnight and driving the following day seems fine (as per my post on page 2).
Also, your words seem to suggest letting the car sit at 100% is the only issue. That is certainly not the case. There are lots of causes of problems with Li batteries - Charging rates of >250kW, Discharge rates of >250kW, Low temp charging, mechanical stability etc, etc. The batteries and their BMS have to be spot on to survive....but they do. The problems of 100% SoC start with the charging to that point iteself as that is when balance problems start. Poor balancing leads to overcharging (or charging problems at the bottom end) to combine with the issues of keeping the SoC at 100% for extended hours.

 

[Tooney]

Description of cell balancing from Audi e-tron training manual

https://www.taycanforum.com/forum/t...iled-technical-information.13372/#post-200398

 

[007]

I can’t believe it, you guys are all spoiled and rude… be a little considerate to those of us that can’t charge at all anymore

Well, I guess it’s my turn. After nearly a year of trouble free ownership, today my car will not charge. It’s a 2021 Taycan Turbo S with 39k km on the odometer.

I first thought it was the home charger (I use the Porsche Mobile charger). I did a factory reset and tried both the 3 phase and the regular 230V wall plugs. Nothing worked . It seems like it almost wants to charge (makes the regular clicking noises when connecting, fan turning on) but after a minute or so of white flashing it errors out, turns red. Tried several times. The charger itself looks ok, screen ok, discovers the car, no error messages.

So I drove to a nearby hotel that has a few free Porsche branded chargers. Tried two different plugs, same thing, didn’t charge. Then I drove to another hotel/restaurant where I happened to charge two weeks ago during dinner without issues, same thing, nothing.

All AC chargers so not sure if a DC one would work, but don’t have one nearby so didn’t check. But either way, need the AC to work.

Car drives like normal, no error messages. I think like others in this thread, that it might be the 22Kw onboard charger or something related to this.

Tow truck just picked it up from home (with a 15% charge left) and will take it to the dealer in Kraków that’s an hour and an half away from here.

Got a Mercedes CLA200 from Porsche Assistance in the meantime which has the power of a go kart.